Iain Duncan-Smith is not an idiot. It’s entirely possible his resignation is purely connected to the Government’s decision to cut billions from disability benefits. But he knows exactly how much damage he is doing to that Government, and personally to both David Cameron and George Osborne.

His resignation letter was made public one hour before the main TV news bulletins on Friday. They will dominate a weekend that should have been about all the other measures in the Budget.

IDS has had his disagreements with George Osborne — and the Quiet Man’s decided now is the time to get his own back.

His letter is damning, saying he is “unable to watch passively” as cuts “perceived as distinctly political” are pushed through.

He speaks of how “too often” he’s been put under pressure immediately before a Budget to find further reductions in working-age benefits, and suggests he’d wanted better-off pensioners to bear more of the burden — something rejected in Numbers 10 and 11.

David Cameron’s response is almost as angry. He is “puzzled and disappointed”, insisting Iain Duncan-Smith agreed the cuts with the Treasury and Downing Street — he is determined not to let him leave government without taking a share of the blame.

And be in no doubt, there’s an awful lot of politics at play here. IDS had already hinted his time in government may be coming to a close, his decision to campaign to leave the EU possibly sealing his fate.

In that case, why not walk out at a time and in a manner of your own choosing, and stick the boot into your enemies as you go.

Critics point out he has presided over more than a few unpopular welfare reforms, which have themselves made some vulnerable people poorer, and suggest the timing is more connected with the EU referendum than concern for those in poverty.

That’s something he denies — but he knows his actions will harm the Government he’s been a part of for 6 years, forever mark the 2016 Budget as a political failure, and damage both the Chancellor and Prime Minister.

There’ll have to be a reshuffle, and the Chancellor’s leadership ambitions, which were meant to be boosted this week, have been battered.

Immediately after the election, George Osborne led a land-grab for the kind of territory normally occupied by Labour — positioning the Tories as the party of ordinary working people. But to those ordinary working people, taking billions off the disabled seems like an odd choice, especially when you’re handing back hundreds of pounds to the highest paid.

Cuts to disability benefits have the power to cause huge damage to the Conservative Party. But the growing rancour around the EU referendum risks tearing the party apart from the inside.

The Tories have form on Europe — an ability to fight long wars, and bear unending grudges, regardless of how they’re viewed by voters. It’s been the party’s Achilles heel since the mid-80s.

Iain Duncan-Smith, one of the original Maastricht rebels of the 1990s, whose actions contributed to John Major’s headaches, will now sit on the back-benches, where he could cause just as much trouble for David Cameron.